The toothy predators shed their bad rap in a new documentary from a Sausalito film company that is screening “Sharks: Stewards of the Reef” at the Lark Theater on Saturday for World Ocean Day. Cameraman and shark advocate David McGuire of San Rafael says Trillium Films’ piece is a big departure from the “slashing, gnashing teeth” footage that dominates programs like the Discovery Channel’s “Shark Week.” Where nature photographers will chum the water to get it churning with sharks, McGuire skipped the blood-in-the water approach. “We wanted to get sharks in their natural environment with their natural behaviors,” McGuire says.

That’s important because the sharks’ presence in their natural habitats is closely linked to the viability of those natural ecosystems, and sharks are being devastated by a most voracious predator on the sea: man. “Sharks: Stewards of the Reef” suggests shark populations have been devastated by overfishing and an exploding demand for shark fin soup.

Once the dish of royalty, the soup creates a devastating demand for the fins. Fishing fleets chasing tuna and other major fish stocks are hooking sharks as “by catch.” The fisherman hack off the fins, which end up in the soup, and toss the shark bodies overboard.

“You go down to Chinatown and there are bins and bins of dried shark fins,” McGuire says.

The film came about as part of an adventure and became an adventure of its own. McGuire accompanied Trillium founders Holiday and Christopher Johnson on a sailboat voyage to the Marquesa Islands in the South Pacific. Holiday Johnson had made a documentary on the fight to save coastal redwoods and Christopher Johnson is the executive director of Ocean Explorer, an education and advocacy group. The South Pacific sojourn quickly turned into a mission.

“We said. ‘We need to make a film,'” McGuire recalls.

The ocean has perhaps too many stories to tell. Beset by overfishing, pollution and global warming, there is no shortage of dire situations. But all of them kept coming back to sharks.

As an “apex predator,” sharks are affected by the vitality of the world around them. All of nature is interconnected, but sharks are a linchpin at the intersection of so many issues.

“This is a crisis,” McGuire says. “This is an ocean crisis.”

But McGuire and the Trillium crew didn’t want to make just another shark film. They wanted to make a film that cuts through the dark pop-cultural imagery and presents sharks as vulnerable – a contrast to their perceived super-predator status. Sharks need better public relations.

“There is no cuddly shark,” McGuire says. “There is still a huge ‘Jaws’ mentality.”

Sonja Fordham of the Ocean Conservancy appears in the film. In an interview by e-mail, Fordham says sharks are clearly misunderstood.

“Sharks are viewed as low-value pests or man-eaters,” Fordham says. “Despite their fierce image, sharks are among the most vulnerable animals in the ocean.”

Rescuing the sharks’ image is much of what “Sharks: Stewards of the Reef” is about. Filming the sharks in chum-free natural settings was a central mission statement for McGuire and Trillium, but so was documenting their plight. Sharks are not well counted. Most fish populations are counted based on what fishing boats are catching. Sharks are not a target species, and the shark fin soup “finning” phenomenon is shadowy.

“It’s like a black market drug trade,” says McGuire, who recalls posing as a tourist in a Honolulu alley, hoping to catch the traders in action.

“They were on to me,” he says. (Trillium resorted to purchasing stock footage of the finning process).

That sip of soup has far-reaching effects.

Sharks are vital to a reef ecosystem that is already beset by an array of human activities. They keep the fish populations in balance and the bits of fish that escape their jaws help feed a web of life that includes the coral and every other creature on the reef.

Sharks, Fordham says, are slow to mature, making the effects of overfishing more pronounced. She counts them among the “species most vulnerable to extinction.”

Those threats can be hard for the public to see. Fishing is far off shore, the fish populations out of sight beneath the waves.

Fordham says “Sharks: Stewards of the Reef” helps make all it more immediate. It’s not “Shark Week.”

McGuire, who is now working on a piece to document the “environmental and societal” impacts of protected marine sanctuaries, says he hopes the film can offer at least a glimmer of hope. The popular image of sharks needs to change, and attitudes and culinary habits need to follow – quickly. The film could be part of that.

“We’re going to translate our film and show it in China,” he says.

In the end, sharks are just one indicator of the environmental stress. What McGuire and the Trillium captured on film is just one piece in a very complicated and globe-spanning mosaic.

“It’s not just about protecting sharks,” McGuire says. “It’s about protecting (eco) systems.”

IF YOU GO

What: The World Ocean Day Film Celebration, featuring a screening of “Sharks: Stewards of the Reef” along with a selection of marine environment films and a surf film